Billing: fatture manuali, profilo obbligatorio e piani Stripe mensile/annuale.

Rimuove link al portale Stripe in area cliente, aggiunge flusso admin per PDF
fattura e email, blocca checkout senza dati di fatturazione, allinea prezzi
a €4,90/€39,90 e €9,90/€69,90, locale italiano e documentazione deploy.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/db/migrate/20260530120000_add_billing_interval_to_subscriptions.rb

@@ -0,0 +1,5 @@
+class AddBillingIntervalToSubscriptions < ActiveRecord::Migration[7.2]
+  def change
+    add_column :subscriptions, :billing_interval, :string
+  end
+end

backend/db/migrate/20260531140000_billing_invoices_manual_flow.rb

@@ -0,0 +1,12 @@
+class BillingInvoicesManualFlow < ActiveRecord::Migration[7.2]
+  def change
+    change_column_default :billing_invoices, :source, from: "aruba", to: "manual"
+    change_column_default :billing_invoices, :status, from: "issued", to: "draft"
+
+    add_column :billing_invoices, :emailed_at, :datetime
+    add_index :billing_invoices, :billing_payment_id,
+              unique: true,
+              where: "billing_payment_id IS NOT NULL",
+              name: "index_billing_invoices_unique_payment"
+  end
+end